1. Field of the Invention
The present invention generally relates to a hybrid construction machine including an engine and an electric motor/generator. More particularly, the present invention relates to a control technique suitable for wheel loaders and similar construction machines, which frequently generate great traction forces at very low speed.
2. Description of the Related Art
Japanese Laid-Open Patent Application Publication No. Hei. 9-14385 discloses a conventional hybrid vehicle including an internal combustion engine and an electric motor, as a driving power source, in which an output shaft of the engine is coupled to a sun gear of an epicyclic gearing, an output shaft of the electric motor is coupled to a ring gear of the epicyclic gearing, and a composite output of the output shafts of the engine and of the electric motor is output to a drive wheel through a planetary carrier.
In this hybrid vehicle, a rotation of the carrier coupled to the drive wheel side stops during a stopped state of the vehicle (i.e., output rotational speed is zero), while idle rotation of the engine is input to the sun gear, causing the ring gear and the electric motor coupled to the ring gear to rotate reversely (in a reverse direction). In this state, the electric motor performs regenerative braking, stops its reverse rotation smoothly, and shifts to a power running mode in which it rotates normally (in a normal direction). Thus, the vehicle smoothly starts.
Unlike a driving pattern of the above mentioned vehicle, wheel loaders and similar construction machines do not frequently drive continuously at constant vehicle speed which is not less than certain speed. A large part of the operation of the wheel loader is a V cycle operation. For example, the wheel loader starts and drives toward dirt (earth) and shovels the dirt. Then, the wheel loader drives backward and then drives forward toward a truck. Thus, the wheel loader accelerates and decelerates repeatedly at low speed. When the construction machine is shoveling the dirt, a great traction force is needed in a very low-speed range, which is referred to as a stall state.
Under the circumstances, typically, wheel loaders of medium and larger sizes are equipped with torque converters. It is well known that the torque converter has a low efficiency as a whole and a very low efficiency particularly in a low-speed ratio. The torque of the torque converter cannot be transmitted sufficiently when engine speed is low. Such a characteristic of the torque converter is not always suitable for the above mentioned driving pattern of the wheel loader such as repeated acceleration and deceleration, stall, etc.
To solve the above problem, inventors studied intensively to provide hybrid construction machines such as the wheel loader which is not equipped with the torque converter, and conceived a structure in which an epicyclic gearing composites a driving power of an electric motor/generator and a driving power of an engine and outputs the resulting composite driving power to drive wheels, and a transmission is provided between the epicyclic gearing and the drive wheel (see Japanese Laid-Open Patent Application Publication No. 2008-247269).
In this construction machine, to attain a driving capability which is equal to or higher than that of the torque converter when a direct-coupling clutch is disengaged in a low-speed range, engine speed is controlled according to a state of charge (SOC) in an electric storage device, an operation of the electric motor/generator switches properly between power running and regenerative braking by controlling upshifting or downshifting of a transmission gear position. This enables the construction machine to drive while performing electric charging and electric discharging repeatedly within a limited electricity capacity of the electric storage device while maintaining a balance between engine torque and electric motor/generator torque. As a result, a good driving capability is attained.